Sulphate attack

Sulphate attack has often been discussed in terms of reaction between solid phases in the cement paste and dissolved compounds, such as Na SO or MgSO, in the attacking solution. This obscures the fact that the reactions of the cations and anions in that solution are essentially independent for example, a solution of Na2S04 may cause both sulphate attack and ASR (T60,P53), and one of MgS04 causes sulphate attack and reactions forming brucite. 

The observed microstructures show that both ettringite and gypsum are deposited from solution, crystallization thus taking place in accordance with the following equations  

The A1(0H)4 and OH , and some of the Ca, needed to form ettringite eould be supplied by iinreaeted aluminate or ferrite phase, but the main souree of Al(OH)4 hardened concrete is likely to be AFm phase, represented here as monosulphate  

Gypsum that has been formed by the reaction shown in equation 12.8 may dissolve with precipitation of ettringite. The dissolution is represented by the reversal of the same equation, and the additional ions required eould be provided by dissolution of monosulphate, represented by equation 12.9. 

C3S pastes are slowly attacked by 0.15 m Na2S04 solution more coneen-trated Na2S04 solutions also attack P-C2S pastes (T60). The action is possibly due to formation of gypsum. Lea (L6) stated that Na2S04 solutions do not attack the C-S-H in cement pastes, but the SEM evidence does not support this view. While the observed decalcification of the C-S-H might be due to carbonation, which may take place more readily in material affected by sulphate attack, it could also be a direct consequence of the latter. C-S H must be attacked in accordance with equation 12.4 if the Ca concentration in the pore solution with which it is in contact falls sufficiently, and the formation of ettringite might produce such a decrease. 

The sulphate attack has been known from a long time, and already in 1858 Vicat [247] studied the chemical causes of hydraulie eompounds corrosion in sea water [247]. Bied [248] invented the teehnology and developed the production of calcium aluminate cement, as a remedy for rapid destmetion of concrete in France, caused by the sulphate ground water attaek, from the dissolution of gypsum and anhydrite. 

The opinion of favourable effect of pozzolanic additions, particularly blastfurnace slag and siliceous fly ash against the srrlphate corrosion, is maintained to the present days and has the universal, rich documentation [251], However, some authors are of the opinion, that the physical effect of reduced porosity and capillary adsorption (permeability), in the case of high performance concrete with superplasticizers addition is more important than the influence of pozzolanic additions [252]. Analogous opinion was supported by Skalny and Pierce [65]. 

In spite of numerous research there are still many umesolved and controversial problems inspiring the intensive discussions. A good example is the problem of ettringite and the mechanism of its expansive action. There are maity hypotheses concerning this problem, but they all are at least doubtful [176], On the basis of considerations presented by Scherer [173], Brown and Taylor [176] are willing to accept the crystalhzation pressure of micrometric particles as a main cause of ettringite expansive effect. Simultaneously, as a rule the expansion of concrete due to the sulphate attack is always linked with the formation of ettringite. 

Similarly, as in the case of the other types of corrosion, the inner and outer concrete corrosion can be distinguish, as it has been mentioned earlier, and, in the case of sulphate attack, this classification can be considered as a classic one. 

The complexity of chemical reactions was underhned by Taylor [255]. There are several mutually related processes, including not only the formation of ettringite and gypsum, decomposition of C-S-H phase through decalcification, but also the precipitation of bracite, occurring in the case of magnesium sulphate they all lead to the destruction of material [176]. 

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Air-entraining admixtures, therefore, produce concrete which is more durable to conditions of freezing and thawing, particularly in the presence of de-icing salts, more resistance to sulphate attack, provides better protection to embedded reinforcement and is more tolerant of poor curing conditions. There appears to be no... 

Boiling, concentrated sulphuric or hydrochloric acid slowly dissolves the metal. Fused potassium hydrogen sulphate attacks palladium. 

Fig. 12.7 SEM (backscattered electron image) of a concrete showing massixe deposits of ettringite (one marked E ), which through sulphate attack has grown on aggregate surfaces, with associated cracking. Courtesy WHD Microanalysis Consultants Ltd, Ipswich, UK (H64).

Corrosion is not the only deterioration mechanism in reinforced concrete. Alkali-silica reactivity (ASR), sulphate attack, thurmasite attack, delayed ettringite formation, freeze thaw, thermal movement, settlement and other movement can all lead to concrete damage and their assessment must be included in the surveys. 

Plasticizers and superplasticizers improve radically the pore stmcture (effect of w/c) and concrete becomes less permeable to air and water [159], Collepardi and Massida [159] found a capillary porosity and pore size lowering with decreasing permeability of concretes in which water reducers were used. The resistance to the sulphate attack was also improved [159]. 

The sulphate attack is more severe as the concrete is subjected to cyclic wetting and drying. Therefore the laboratory expansion measurements do not reflect completely the field conditions. Mehta [251], basing on the observations of various concrete structures, foimd that the decrease of adhesion and strength, as well as the... 

The thaumasite CaSO CaSiOj CaCOj 15H2O [257, 258] appears often, apart from gypsum and aluminum hydroxide or silica gel, in the deteriorated concrete. The formation of this phase takes place when the sulphate attack occurs at low temperature and together with an intensive carbonation process. 

The role of the ferrite phase, generally identified as brownmillerite, should be mentioned too. In the case of sulphate attack this phase can be the source of almninate ions [237] moreover the ferrite ions can form the analogue of ettringite or to substitute the aluminate ions in all calcium aluminate phases [222]. The latter case is undoubtedly the most common one in the Portland cement paste. However, the reaction of sulphate ions with ferrites is slower. There is a view that the F/Al ratio in the hydrated phases is lower than in brownmillerite hence, some amount of iron(in) hydroxide is always present [222] (see also Sect. 4.1.1.). This hydroxide occurs in the gel-like form and therefore the diffusion of ions through the gel layer is slowed down. Therefore, the corrosion process is hindered. The other phases containing the Fe ions can be produced too, it is discussed in Chap. 3. 

As it has been mentioned earUer, during the sulphate attack, as in the case of the other corrosion processes, the zonal phase composition changes of the paste, with the decreasing concentration of sulphate ions, in the direction to the concrete interior along the diffusion path, is observed. Gollob and Taylor [256] studied the... 

Fig. 6.63 Appearance of normal features not subjected to sulphate attack A—unhydrated cement, B—dense inner C-S-H gel surrounding unhydrated cement, C—inner C-S-H gel constituting fully hydrated cement grain, D— region of small hollow shell hydration grains, E—groundmass or outer product C-S-H gel, F—Ca(OH)j surrounding a sand grain chip, G—deposit of calcium hydroxide within the groundmass (After [261]) Diamond S., Lee R.J. in Materials Science and Concrete, Special volume Sulfate Attack Mechanisms (J. Marchand amd J. Skalny eds.), p. 138, Fig. 1, 1999, published by The American Ceramic Society, 735 Ceramic Place, Westerville, Ohio 43081, 2001, reproduced with the permission of The American Ceramic Society...

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